Air-water Flow Downstream of High-Head Sluice Gates: Experimental and Numerical Investigations

Air entrainment is a typical phenomenon encountered in high-speed flows in hydraulic engineering applications. Strong interface deformations and high turbulence intensities lead to complex air-water flow features. High-speed air-water flows on steeply sloping spillway chutes are well studied and typical macroscopic two-phase flow properties like inception point location, mean air concentration and interfacial velocity can be described with reasonable accuracy. However, such information is limited for high-speed flow on small to moderately sloped channels typically encountered in high-head outlet structures. Therefore, this study aims to improve the understanding of high-velocity air-water flows downstream of a high-head sluice gate.

The effect of different inflow conditions, such as Froude number at the contraction (‘vena contracta’), on air-water flow properties was experimentally investigated. The experiments were conducted in a scale model at VAW, ETH Zurich. The sharp-crested gate in the model was followed by a 0.2 m wide, 0.3 m high closed-conduit with a total length of 20.6 m and slope of 4%. A wide range of flow conditions was tested using phase-detection intrusive probes to measure the air-water flow properties. The obtained data provided novel insights into high-velocity air-water flows and were also used for the calibration of a numerical model. A Reynolds-averaged mixture model approach was used to simulate the flow conditions observed in the hydraulic model. Air entrainment was considered based on the turbulent kinetic energy at the air-water interface and the relative bubble movement was reflected by a drag-based drift-flux model.

The longitudinal development of the depth-averaged air concentration showed two different air entrainment processes. Close to the gate, air entrainment was dominated by shockwave formation and shear forces at the air-water interface, while further downstream the process was dominated by the turbulent boundary layer reaching the free surface. For small gate openings this led to two distinct peaks in the depth-averaged air concentration, while for large gate openings only the second peak, driven by turbulence, was observed. The simulated mixture surface and air concentration profiles showed an overall good agreement with the measurements. However, close to the gate significant deviations were observed for some configurations, indicating that the numerical model was only able to capture the second, turbulence driven air entrainment process. Overall, the general knowledge on air-water flows downstream of high-head sluice gates was advanced and the capabilities and limitations of numerical simulations of such flows were demonstrated.